The invention relates to a method for the operation of an internal combustion engine which can be operated selectively in a compression ignition mode and in a spark ignition mode.
To achieve reduced fuel consumption, 4-stroke internal combustion engines can be operated in different operating modes. In one operating mode with compression ignition, a high efficiency engine operation and, consequently, a low fuel consumption are established. Under corresponding operating conditions with a suitable temperature and with a corresponding cylinder pressure, a homogeneous air/fuel mixture is subjected to autoignition with correspondingly favorable combustion conditions.
Compression ignition operation, however, is restricted to the part-load range, since, with a rising load, the maximum cylinder pressure also rises and may overshoot a permissible value. This results in increased noise emission and in an undesirably high load on the engine components. With higher rotational speeds, furthermore, the knocking tendency of the cylinder charge increases in an undesirable way. A further disadvantage is the exponential rise in the formation of nitrogen oxides as the maximum process temperature rises as a result of excessively high load.
To avoid the disadvantages mentioned, a corresponding internal combustion engine is operated in the higher load range in a second operating mode with conventional spark ignition. In this case, the known Otto-cycle combustion takes place, which is suitable for operation under higher loads.
Particularly in a motor vehicle an internal combustion engine is exposed to changing operating loads. A change between the two operating modes is desired as a function of the prevailing load.
The change between two operating modes may be brought about by a cyclic change in the control times of the corresponding inlet and outlet valves. For this purpose, electromechanical valve controls (EMVS) or electrohydraulic valve controls (EHVS) are known, by means of which the opening and closing times of the inlet and outlet valves can be adapted, as required. By a variation in, for example, the outlet control times, the outlet valve can be closed comparatively early. This causes exhaust gas retention in the cylinders, as a result of which an increased pressure and an increased temperature are established, which lead to compression ignition.
U.S. Pat. No. 6,336,436 discloses a method for the operation of a 4-stroke internal combustion engine, in which a change can be made, as required, between compression ignition operation and spark ignition operation. Adaptation of the valve control times to the respective operating mode by means of a variably adjustable valve drive is proposed. The operating conditions for compression ignition operation with autoignition are set via exhaust gas recirculation.
A disadvantage which became apparent is that, during the change from spark ignition operation to compression ignition operation, while the operating parameters otherwise remain unchanged, an undesirably increased noise emission occurs.
It is the object of the present invention to provide a method by which the changeover between a compression ignition operating mode and a spark ignition operating mode is facilitated.